1. Field of the Invention
The invention relates generally to the field of passive seismic surveying. More specifically, the invention relates to methods for deploying sensors and acquiring passive seismic data to reduce noise and enhance ability to identify microseismic events.
2. Background Art
Passive seismic emission tomography is a process in which an array of seismic sensors is deployed in a selected pattern on the Earth's surface (or on the water bottom in marine surveys) and seismic energy is detected at the sensors that emanates from various seismic events occurring within the Earth's subsurface (“microseismic events”). Processing the signals detected by the sensors is used to determine, among other things, the position in the Earth's subsurface at which the various seismic events took place.
Applications for passive seismic emission tomography include, for example, determining the point of origin of microearthquakes caused by movement along geologic faults (breaks in rock layers or formations), movement of fluid in subsurface reservoirs, and monitoring of movement of proppant-filled fluid injected into subsurface reservoirs to increase the effective wellbore radius of wellbores drilled through hydrocarbon-producing subsurface Earth formations (“fracturing”). The latter application, known as “frac monitoring” is intended to enable the wellbore operator to determine, with respect to time, the direction and velocity at which the proppant filled fluid moves through particular subsurface Earth formations.
Passive seismic emission tomography for the above types of interpretation includes determining what are seismic-induced events from within the signals detected at each of the seismic sensors, and for each event detected at the seismic sensors, determining the spatial position and time of the origin of the seismic event.
In applications for passive seismic emission tomography related to reservoir and/or frac monitoring, typically the dominant sources of noise occur proximate the Earth's surface. One technique for reducing such noise is to place the each of the sensors in a wellbore drilled through near-surface layers of sediment and rock. By placing the sensors in wellbores, typically at depths of about 100 meters, surface occurring noise can be reduced in the detected seismic signals. Even using such technique, there is a need for improved surface occurring noise reduction to enable detection of small amplitude microseismic events. Generally, surface noise diminishes with depth of the sensor and the signal increases as the sensors are closer to the events, resulting in increased signal-to-noise ratio. Therefore the most common installation of such monitoring sensors is to deploy one geophone into each borehole as deep as possible. Even at such greater depths, there is still a need for improved noise reduction in passive seismic sensing arrays.